A mammalian eye includes a cornea and sclera. The sclera provides a structure for the eye that gives the eye a generally spherical shape. The sclera also gives the major surface portion of the eye its white color. The cornea is a transparent front part of the eye that covers an iris, a pupil, and an anterior chamber that is disposed in front of a lens. Light passes through the transparent cornea and then through the pupil to fall upon a retina that senses the passed light. Together, the retina and a brain produce vision. Clinicians are concerned with the proper function and health of the eye.
The function of the eye can be affected by aberrations to the shape of the cornea. Therefore, clinical diagnosis of vision will benefit from capturing and displaying a corneal topography image. In essence, corneal topography is a non-invasive procedure used to determine the shape and integrity of the cornea of the eye. During a corneal topography, a clinician projects a series of illuminated rings known as a Placido pattern onto the surface of the cornea. The Placido pattern is reflected back into a computerized camera system. Typically, the computerized camera system analyzes the reflected Placido pattern to generate a topographical map of the cornea. The resulting corneal topographic images are analyzed by the clinician to determine the health of the eye. For example, corneal topography is used to analyze corneas before and after vision correction surgery and for contact lens fitting, etc. It is known that a contact lens fitting that is too tight interferes with natural tear flow. Therefore, it is important to provide a precorneal tear film analysis after a contact lens fitting.
In this regard, clinical analysis of the precorneal tear film can be provided to improve eye health and provide comfortable vision. In the human eye, the precorneal tear film covering ocular surfaces is composed of three primary layers: the mucin layer, the aqueous layer, and the lipid layer. Each layer plays a role in the protection and lubrication of the eye and thus affects dryness of the eye or lack thereof. Dryness of the eye is a recognized ocular disease, which is generally referred to as “dry eye,” “dry eye syndrome” (DES), or “keratoconjunctivitis sicca” (KCS). Dry eye can cause symptoms, such as itchiness, burning, and irritation, which can result in discomfort. There is a correlation between the ocular tear film layer thicknesses and dry eye disease. The various different medical conditions and damage to the eye, as well as the relationship of the aqueous and lipid layers to those conditions are reviewed in Sury Opthalmol 52:369-374, 2007 and additionally briefly discussed below.
As illustrated in FIG. 1, the precorneal tear film includes an innermost layer of the tear film in contact with a cornea 10 of an eye 12 known as the mucus layer 14. The mucus layer 14 is comprised of many mucins. The mucins serve to retain aqueous matter in the middle layer of the tear film known as the aqueous layer. Thus, the mucus layer 14 is important in that it assists in the retention of aqueous matter on the cornea 10 to provide a protective layer and lubrication, which prevents dryness of the eye 12.
A middle or aqueous layer 16 comprises the bulk of the tear film. The aqueous layer 16 is formed by secretion of aqueous matter by lacrimal glands 18 and accessory tear glands 20 surrounding the eye 12, as illustrated in FIG. 2. The aqueous matter, secreted by the lacrimal glands 18 and accessory tear glands 20, is also commonly referred to as “tears.” One function of the aqueous layer 16 is to help flush out any dust, debris, or foreign objects that may get into the eye 12. Another important function of the aqueous layer 16 is to provide a protective layer and lubrication to the eye 12 to keep it moist and comfortable. Defects that cause a lack of sufficient aqueous matter in the aqueous layer 16, also known as “aqueous deficiency,” are a common cause of dry eye. Contact lens wear can also contribute to dry eye. A contact lens can disrupt the natural tear film and can reduce corneal sensitivity over time, which can cause a reduction in tear production.
The outermost layer of the tear film, known as the “lipid layer” 22 and illustrated in FIG. 1, also aids to prevent dryness of the eye. The lipid layer 22 is comprised of many lipids known as “meibum” or “sebum” that are produced by meibomian glands 24 in upper and lower eyelids 26, 28, as illustrated in FIG. 3. This outermost lipid layer is very thin, typically less than 250 nanometers (nm) in thickness. The lipid layer 22 provides a protective coating over the aqueous layer 16 to limit the rate at which the aqueous layer 16 evaporates. Blinking causes the upper eyelid 26 to mall up aqueous matter and lipids as a tear film, thus forming a protective coating over the eye 12. A higher rate of evaporation of the aqueous layer 16 can cause dryness of the eye. Thus, if the lipid layer 22 is not sufficient to limit the rate of evaporation of the aqueous layer 16, dryness of the eye may result.
Notwithstanding the foregoing, it has been a long standing and vexing problem for clinicians and scientists to quantify the lipid and aqueous layers and any deficiencies of same to diagnose evaporative tear loss and/or tear deficiency dry eye conditions. Further, many promising treatments for dry eye have failed to receive approval from the United States Food and Drug Administration due to the inability to demonstrate clinical effectiveness to the satisfaction of the agency. Many clinicians diagnose dry eye based on patient symptoms alone. Questionnaires have been used in this regard. Although it seems reasonable to diagnose dry eye based on symptoms alone, symptoms of ocular discomfort represent only one aspect of “dry eyes,” as defined by the National Eye Institute workshop on dry eyes. In the absence of a demonstrable diagnosis of tear deficiency or a possibility of excessive tear evaporation and damage to the exposed surface of the eye, one cannot really satisfy the requirements of dry eye diagnosis.